The insertion of unmanned aircraft, or drones, into the airspace is currently limited to appropriate, so-called “segregated”, spaces so as to limit the risks of incidents. Segregated spaces are reserved spaces, often military, which are subject to specific rules, and into which civil equipment, such as civil or commercial aviation aircraft, do not enter.
The opening up of non-segregated airspace to drones poses numerous technological problems, such as that of the reliability of anticollision mechanisms, or that of the reliability of the telecommunication systems used for control and command. The telecommunication systems used will therefore have to meet strict criteria in terms of performance (packet error rates, latency of the network, useful bitrate, continuity and availability of service).
In order to meet these heavy requirements, it is possible for the communication means to be made redundant, by using for example a satellite main network and a terrestrial secondary network.
Two strategies for managing the redundancy of the communication means are then possible.
The first strategy consists in transmitting via a single main network and in switching the communications over to a secondary network in case of failure. This solution requires the detection of the failure of the main network, and the re-establishment of the current communications on the secondary network.
The drawback of such a solution is that the system is unavailable during the failure detection time and during the time for re-establishment of the communication via the secondary network. The availability of the system is therefore degraded. Depending on the duration of service interruption, the failure can be considered to be a loss-of-service-continuity event.
Moreover, depending on the redundancy mechanism implemented, the packets currently being transmitted via the main network may be lost and will have to be retransmitted by the secondary network. The failure will therefore lead to more considerable transmission latency. The latency jitter will therefore also be more considerable.
Finally, depending on the redundancy mechanism chosen, the re-establishment of the communications on the secondary network may require protocol exchanges with the terminals. In this case the re-establishment of the communications is not transparent to the users. As all the users of the main network are impacted by the failure, the communications re-establishment mechanism will cause a spike in usage of the network in respect of the protocol exchanges. Depending on the dimensioning of the system, this spike may give rise to a choking of the system and in all cases will lead to a more considerable latency.
This strategy is therefore not suitable for the control and command of drones.
The second strategy consists in transmitting the traffic in parallel via the two networks, and in deleting the duplicate blocks on reception. The advantage of such a solution is that in case of failure of one of the networks, the communication is not interrupted and continues via the available network. Service availability and service continuity are therefore not impacted by the failure of one of the networks. The failure of a network is transparent to the users and does not require any protocol exchanges.
The drawback of this second solution is that in nominal mode it requires twice as much bandwidth over the air and in the networks.
So as to limit the occupancy of the bandwidth, and to reduce the cost of the equipment, it is known to use the same frequencies to communicate with the satellite network and the terrestrial network.
The document “Hybrid Satellite/Terrestrial Cooperative Relaying Stategies for DVB-SH based Communication Systems” proposes using the DVB-SH (Digital Video Broadcasting-Satellite Handheld) standard in a hybrid system composed of a satellite network, and of one or more terrestrial networks responsible for relaying, on the same frequency, the message transmitted by the satellite network.
The DVB-SH standard uses a waveform of OFDM (Orthogonal Frequency-Divison Multiplexing) type associated with turbo codes and with an MRC (Maximum Radio Combining) mechanism so as to recombine the signals at the level of the user terminal.
The signal transmitted from the terrestrial network is delayed with respect to the signal transmitted from the satellite network. The use of OFDM modulation makes it possible to compensate for the transmission delays, and to recombine the two signals.
However, this mechanism is not suitable for access modes requiring accurate synchronization between the signals, such as TDMA (Time Division Multiple Access), or WCDMA (Wideband Code Division Multiple Access).
Moreover, the delay between the signal transmitted by the satellite network and the signal transmitted by the terrestrial network may moreover be considerable. This delay is compatible with the use of OFDM modulation, but not necessarily with waveforms having more limited possibilities for compensating for delays, such as waveforms with temporal equalizer, or spread waveforms associated with Rake receivers.
The compatibility of a satellite/terrestrial hybrid communication system with access modes of TDMA or WCDMA type, or the use of modulations other than OFDM modulations, therefore requires the synchronization of all of the networks used, in such a way that the messages transmitted by the terrestrial network and the satellite network arrive in a synchronous manner at the user terminal.
The GSM (Global System for Mobile Communications) standard proposes a mechanism for synchronization in a network comprising a base station and a plurality of users. For this, a time shift that has to be applied by each of the user terminals is calculated by the base station, in such a way that the transmitted signals are synchronized when they reach the base station.
However, such operation is incompatible upon the addition of a second network, since this would make it necessary to apply two different time shifts at the level of the user terminals.
Likewise, implementing such a synchronization mechanism at the level of a user terminal would exclude the implementation of several user terminals in the same network.